Move direct calls to MessageSign into new SignMessage functions in CWallet and ScriptPubKeyMan

Summary:
Instead of getting a SigningProvider and then going to MessageSign,
have ScriptPubKeyMan handle the message signing internally.

Backport of Core PR18115 part [7/9] : https://github.com/bitcoin/bitcoin/pull/18115/commits/6a9c429084b40356aa36aa67992da35f61c2f6a2

Depends on D8101 and D8098

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8102